Single-inductor-multiple-output (SIMO) DC-to-DC converter is an important component for portable electronic devices such as cell phone or personal digital assistant (PDA), which requires different supply voltages that are low cost, high efficiency and small in size. However, one major limitation of conventional SIMO converter is cross-regulation. In other words, the outputs of the converter cannot be regulated independently and any load change in one output will affect the others. This is more severe when a large change occurs to the load currents.
Among existing SIMO converter implementations, time-multiplexing control methods suffers from limited power capacity and cross-regulation during large load transient even operating in pseudo-continuous-conduction-mode (PCCM). The operation of this type of SIMO is shown in FIGS. 1(a)-(c) as described in D. Ma et al., “Single-inductor-multiple-output switching converters with time-multiplexing control in discontinuous conduction mode,” IEEE J. of Solid-State Circuits, vol. 38, no. 1, pp. 89-100, 2003, which is hereby incorporated by reference in its entirety and for everything it describes therein. For small cross-regulation, if the given phase is unable to handle the required charge for the corresponding output and during load transient, the cross-regulation will become serious as shown in FIG. 1(c).
It has been proposed to use comparator controlled output control to reduce cross-regulation by fast response of the comparator controlled output. The operation of this conventional technique is shown in FIG. 2(a)-2(c), as further described in H. P. Le et al., “A Single-Inductor Switching DC-DC Converter with 5 Outputs and Ordered Power-Distributive Control,” IEEE Int. Solid-State Circuits Conf., pp. 534-620, 2007, which is hereby incorporated by reference in its entirety and for everything it describes therein. However, the response time is limited by the last and additional pulse-width-modulated (PWM) controlled output stage, a larger cross-regulation is expected for large load current change as the regulation of the two outputs is not independent to each other, and the output might have a low accuracy due to the nature of the comparator control. FIG. 2(c) shows the cross-regulation when there is a load change at the first output. In this situation, the last output stages receive no charge at several cycles. This problem becomes worse when the number of output stages increases and the PWM controlling the output stage cannot response promptly.
A charge-control technique with large power capacity operating in continuous-conduction-mode (CCM) is described in A. Pizzutelli et al., “Novel control technique for single inductor multiple output converters operating in CCM with reduced cross-regulation,” in IEEE Applied Power Electronics Conference and Exposition, pp. 1502-1507, 2008, which is hereby incorporated by references in its entirety and for everything it describes therein. This system has similar operation principle as the comparator-controlled output control. In there, the cross-regulation is still significant and the technique is only suitable for implementation in buck converter.